This is an open label study of the effect of Sorivudine in the treatment of acute localized zoster and the effect of this drug on levels of dihydropyrimidine dehydrogenase found in leukocytes. During the ten days of therapy, patients will be seen on days 1, 2, 5, 7, and 10; following treatment they will be seen on days 14, 17, 24, 28,31, and 38.